There is a need for differential pressure transducers in general, and especially differential pressure transducers capable of operating at high pressures and high temperatures. U.S. Pat. No. 4,695,817, entitled “Environmentally Protected Pressure Transducers Employing Two Electrically Interconnected Transducer Arrays” by A. D. Kurtz, et al., issued on Sep. 22, 1987 and assigned to the assignee herein, Kulite Semiconductor Products Inc., evidences the need for such pressure transducers, especially for use in external environments, which impose difficult operating conditions for transducer structures. The entire disclosure of U.S. Pat. No. 4,695,817 is hereby incorporated by reference herein.
For example, differential pressure transducers have uses in aircraft, automobiles and other vehicles. In such implementations, the transducer is typically exposed to moisture, fuel, solvents, hydraulic fluids and the elements in general. These transducers are often associated with pressure ports, as well as with internal cavities. During operation, the cavities, as well as the ports, may accumulate excessive amounts of water. Water can harm silicon and metal elements employed in conventional transducers. Metal diaphragms which are conventionally employed either as isolation diaphragms or with strain gauges mounted on an interior surface provide the required media isolation. Such an approach is satisfactory for absolute or sealed gate transducers, however, for gauge or differential transducers, severe problems arise.
The '817 patent describes devices utilizing oil filled sensing cavities for performing differential measurements. These devices rely on the use of PN junction based sensing elements, with cup shaped deflecting membranes. Such a device can be seen, for example, in FIG. 6 of the '817 patent. Basically, a metal diaphragm is used on a front side, behind which a piezoresistive pressure sensor is located within a hollow region filled with oil. The oil acts as an incompressible fluid transmitting the stress applied to the metallized isolation diaphragm to the sensing element. One transducer structure contains a half bridge intended to measure pressure applied to a positive port, while the second transducer structure is comprised of the half bridge intended to measure the negative pressure applied to the negative port. A full bridge is realized by electrically combining the half bridges from the positive and negative ports. Such a device has inherent limitations though.
For example, such a device may be limited to use in relatively low temperatures, such as temperatures below 175° C., due to the inherent temperature limitations of the PN junction. Also, there is no provision for preventing excessive deflection of the sensing membrane, in the event of an overpressure, which could lead to failures in actual use. Even under normal operating conditions, the performance of the PN junction based sensing element is limited due to thermal errors associated with the changes in zero offset and in sensitivity as a function of temperature.
Hence there is a need for an improved pressure transducer which does not utilize PN junction based devices, and which is capable of operating at high temperatures and high pressures. It is a further desire to produce such a transducer which is capable of withstanding high overpressures while maintaining extremely high resolution capability.